Application Note
CHEMICAL
INCOMPATIBILITIES
AFFECTING LEDS
 PREVENTING MISUSE AND DAMAGE TO LED MODULES
DUE TO CHEMICAL INCOMPATIBILITY
This application note aims to provide some level of guidance on the
impact of chemicals and other substances involved in the assembly,
construction and installation of LED-based lighting fixtures.
Introduction
In the interest of both protecting the LED chip and at the same time
maximising the light that is coupled out, nearly all LEDs feature some
form of encapsulation, e.g. a silicone coating for COB or silicone lenses
for SMD. Care must be taken to prevent these silicone compounds from
reacting with any incompatible chemicals or substances, either directly
or indirectly. The chemical structure of the cured silicones used in the LED
manufacturing process is characterised by weak bonds and interstitial
spaces within the solid material (porosity), as a result of which these
silicones are highly susceptible to the diffusion of gas molecules. Cured
silicones are therefore highly permeable to gases such as oxygen, halogens, sulphur compounds and, in particular, volatile organic compounds
(VOCs), which can easily diffuse into the material and then remain there,
causing LED discolouration and degradation.
VOC molecules
Silicone permeability allows VOC
gas molecules to
diffuse into the
dome
LED Chip
Silicone dome
Figure 1: Permeability of the LED's potting agent to gaseous components
due to the high porosity of cured silicones

IMPORTANT
The chemical incompatibility described above affects
all kinds of LED module, e.g. LUGA COB as well as
all modules featuring high-power SMD LEDs. The
degree to which individual modules will be sensitive
to different chemicals and VOCs is currently
unknown!
Foto: Leipziger Leuchten
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Chemical Incompatibilities Affecting LEDs
Incompatible chemicals can originate (outgas) from a variety of materials,
such as adhesives, solder fluxes, gaskets, conformal coatings or potting
materials used in the final product (luminaire). While some of these
chemicals will seriously damage silicone, causing irreversible erosion,
cracking or surface damage, others (particularly many VOCs) will not
actually chemically react with silicone materials, but will rather diffuse into
the silicone and, once lodged inside, will partially oxidise and caused
marked discolouration of the cured silicone. Observable during normal
LED operating conditions (high temperatures, light radiation), this effect
may cause a noticeable shift in chromaticity and a rapid decrease in
luminous flux. Figure 3 clearly illustrates both possible effects:
 WHY AND HOW LEDS DISCOLOUR
1. LED exposed to ambient air.
2. Sealed system without air circulation
due to secondary optics attachment.
3. Trapping of generated incompatible
chemicals
Initial state
4. Any generated incompatible chemicals get trapped.
5. Discoloured layer appears, causing
light degradation (turns brown).
Figure 5: Schematic of an LED displaying the discolouration effect,
caused by incompatible chemicals due to airtight operating conditions.
Luminous flux degradation
Shift in chromaticity
Figure 3: Discolouration of LED Module (WU-M-425) due to high VOC
contamination
Airtight conditions
This discolouration effect is favoured by physically closed systems, which
means spaces without air movement. Operating LEDs within such airtight
systems causes temperatures to rise, which in turn leads to faster vaporisation of incompatible chemicals and their diffusion into the entire closed
system, resulting in discolouration. In this regard, the incompatible chemicals themselves may originate (outgas) from almost all of the components
and materials from which the LED module is made. Possible causes can
include the PCB material or coating, secondary optics, embedded materials (e.g. gaskets), sealants (e.g. glue) and even some LED materials.
Wire Bond
VOC saturated ambience
Silicone Lens
Example of light output degradation
Figures 6 and 7 show a street luminaire fitted with four WU-M-425
modules, notably operated within a hermetically sealed fixture. Figure 6
shows light output at initial power on and Figure 7 represents the same
luminaire’s light output after fewer than 100 hours of operation. The LED
modules display a significant drop in light output with a concomitant
chromaticity drift.
Figure 6: Light output at initial power
of a luminaire fitted with LED modules in an airtight system.
Discoloration
Secondary Optics
LED Chip
Embedded materials
Sealant
PCB
Figure 4: Influence of airtight conditions on discolouration

IMPORTANT
The only possible countermeasure is to ensure
sufficient airflow!
Figure 7: Light output after operating the LED-based, airtight luminaire for
96 hours.
The greater the airflow, the lower the risk of discolouration becomes since any incompatible chemicals
that are generated will be prevented from permanently contaminating the LED module.
Vossloh-Schwabe Deutschland GmbH · Hohe Steinert 8 · 58509 Lüdenscheid · Germany · Phone +49 (0) 23 51/10 10 · Fax +49 (0) 23 51/10 12 17 · www.vossloh-schwabe.com
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Chemical Incompatibilities Affecting LEDs
How VS' IP 66/67 modules prevent discolouration?
 REVERSIBILITY
VS has developed a patented solution that involves a "periscope" on the
optical lens system, which ensures air exchange without compromising the
IP protection class. It is nevertheless essential to ensure that these
IP 66/67-rated VS modules are
not installed in airtight fixtures since
additional sealant coming from the
fixture would prevent air circulation
over the periscope.
The discolouration of LEDs due to the diffusion of incompatible chemicals is a reversible physical process. The LED itself can be recovered by
a change from a sealed to an open system. Operation in an ambient
atmosphere may allow the oxidized chemical molecules to diffuse out
of the silicone and may restore the initial optical properties of the LED
component, meaning that the discolouration will disappear.
Incompatible chemicals may even completely outgas from the interior of
the silicone potting material if no chemical reaction has taken place with
any part of the LED (no contamination).
Wire Bond
Figure 8 (below) shows the discolouration of an LED caused by the diffusion of incompatible chemicals.
Clear Chip
Silicone Lens
Discoloration
Lens Cover
LED Chip
VOC Source
Sealant
Discolored Chip
PCB
Figure 8: Examples of a normal (left) and a chemically contaminated LED
(right)
Airtight System
The discolouration will be most pronounced at the point with the highest
temperature, which is primarily on the surface of the LED chip during
normal operating conditions.
The likelihood and degree of discoloration will depend on the type of
chemical, its concentration, the temperature during exposure and how
long the module is exposed to the chemical. The permeability of silicone
increases with higher temperatures, which facilitates the – mostly reversible – chemical exchange process and so leads to more incompatible
chemicals diffusing into or being evaporated out of the silicone potting
material.
VOC molecule
evaporates
Outgassing VOC
Open System
Figure 10: Airflow in the system permits any trapped incompatible chemicals to escape from the LED
Depending on the nature of the trapped incompatible chemicals,
e.g. the size of molecules or their sensitivity to heat, discolouration can
occur within a few hours or only after several weeks. The same is true for
the clearing (outgassing) process.
VOC molecule

IMPORTANT
However, certain incompatible chemicals can destroy
the LED, commonly causing the potting material to
swell and crack, which in turn leads to unacceptable
light output.
Figure 9: Outgassing of incompatible chemical molecules inside an
open system permitting normal air circulation.
Vossloh-Schwabe Deutschland GmbH · Hohe Steinert 8 · 58509 Lüdenscheid · Germany · Phone +49 (0) 23 51/10 10 · Fax +49 (0) 23 51/10 12 17 · www.vossloh-schwabe.com
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Chemical Incompatibilities Affecting LEDs
Experiment demonstrating process reversibility
Silicone Lens
Wire Bond
Discoloration
LED Chip
Lens Cover
Sealant
After 100 hours of VOC-free operation, the LED has regained 100% of its
initial luminous flux.
VOC Evaporation
Recovery of luminous flux by
allowing airflow
Decrease of luminous flux
due to evaporated gas in
airtight conditions
1
2
3
4
5
6
0h
20 h
40 h
60 h
80 h
100 h
Figure 12: Outgassing progression of VOC-degraded LEDs over
100 hours of operation
Relative flux (%)
110
Further example of process reversibility process based on
a WU-M-425 module
100
90
80
Following initial module installation inside an airtight system (sealed
fixture), sufficient air circulation was then ensured inside the fixture.
70
60
50
40
30
20
10
0
10
20
30
0
Operating time (hours)
40
50
60
70
80
90
100
Venting moment
Standard conditions
Airtight conditions
Airtight & vented
Figure 11: Typical behaviour of VOC-contaminated LEDs and LED
modules
Basic points concerning the shown typical behaviour curves:
three identical LEDs modules were first contaminated with a known incompatible chemical substance for a specific period of time, after which the
luminous flux of the modules, each placed within a different environment,
was tested. One LED module was placed in an ambient air environment,
whereas the other two were placed inside airtight systems (sealed with
secondary optics or a glass plate). One of these sealed LED modules
was vented after 50 hours, while the other remained sealed during the
entire experiment.
The LED module operated in ambient air displayed no decrease in
luminous flux compared to the initial value despite its high chemical contamination, while the luminous flux of the sealed LED modules decreased
by nearly 90% over the initial values due to discolouration. However, after
the third LED module was vented, thus allowing trapped incompatible
chemicals to outgas, it almost fully regained its previous luminous flux.
Figure 12 illustrates the intermediate steps of process reversal. Picture 1
shows a strongly VOC-discoloured LED operated within a VOC-enriched
environment, whereas pictures 2–6 demonstrate the VOC outgassing
process up to the fully regenerated state when operated in freely circulating ambient air.
Figure 13: Outgassing progression of a VOC-degraded WU-M-425
module over 100 hours of operation
A video showing the regeneration process of a WU-M-425 module can
be viewed on our website at www.vossloh-schwabe.com.
Vossloh-Schwabe Deutschland GmbH · Hohe Steinert 8 · 58509 Lüdenscheid · Germany · Phone +49 (0) 23 51/10 10 · Fax +49 (0) 23 51/10 12 17 · www.vossloh-schwabe.com
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Chemical Incompatibilities Affecting LEDs
Figure 14 illustrates another kind of typical degradation behaviour in LED
modules fitted with numerous single LEDs, in which regard the degradation process itself is random. The first picture shows a section of a sealed
luminaire, only some of whose LED modules have discoloured (not all –
this is a random effect). Once the fixture had been vented and the initial
luminous flux of each LED had been restored (24 hours later), the fixture
was again sealed in an airtight system and operated for a second cycle.
After a while, the modules again began to discolour (final picture), only
this time some of the LEDs that previously discoloured had now retained
their initial state, while others that had remained unaffected during the first
cycle now displayed the discolouration effect. However, sooner or later
all LEDs will degrade if they remain inside an airtight environment that is
enriched with incompatible chemicals.
After several hours in hermetically sealed fixture
first cycle
Fixture opened
Venting
24 hours later
Fixture sealed
Airtight
VS recommends the following precautions for luminaire
design:
• Any potentially outgassing materials that will be incorporated into
the luminaire should be taken into account as an integral part of the
luminaire design so as to understand the environment within which the
LED module will be operated during its service life.
• Avoid using adhesives (especially epoxy-based adhesives), potting
compounds, coatings and undercured materials. Where possible avoid
the use of gaskets or sealants, since incompatible chemicals could be
outgassed from these. If using such sealants or gaskets proves to be
indispensable, it is advisable to use minimal quantities of a siliconecompatible adhesive or to undertake exhaustive compatibility testing.
• Avoid nonpolar fluids and solvents generally used during the luminaire
manufacturing process in such steps as cleaning, oil-assisted drilling
or any processes that would allow the LED array to come into contact
with said fluids or solvents.
• Use only mechanical means of attaching further components such as
lenses, cover plates, circuit boards, etc..
• Do not hermetically seal the lighting fixture. Should sealing the system
prove to be unavoidable (e.g. to satisfy a certain IP protection class),
the following should be observed:
• provide a sufficiently large air gap directly above the LED components
to allow unhindered ventilation (outgassing) of hot air away from the
immediate vicinity of the LED modules.
• Ensure air exchange inside the luminaire e.g. by using gaskets made
of so-called breathable materials or by including one or more valves.
There are many different suitable solutions available on the market. In
any given case, the best choice for the respective fixture design will be
the one that ensures the highest transfer volume.
As an example, the luminaire can be vented by using a GORE membrane, which satisfies IP-class requirements and improves air exchange.
Go to www.gore.com for further details.
A few hours later
Figure 14: Random discolouration
How can the effect be avoided?
The temporary discolouration effect is mainly caused
by incompatible chemicals originating from
surrounding materials. Regardless of the material type
IMPORTANT
(adhesive, conformal coating, potting compound,
etc.), most of the ones used in LED luminaires will outgas during the curing
process. The effect will be drastically accelerated in sealed systems (with
any incompatible chemicals trapped inside) that reach elevated
temperatures. To minimise the negative impact of this chemical incompatibility, careful advance consideration must be given to the choice of
materials and whether there is a need for a hermetically sealed fixture in
order to protect the LEDs from the outside environment.

Figure 15: GORE membranes
.
INFO
All materials used must be fully qualified prior to placing a product on the market.
Vossloh-Schwabe Deutschland GmbH · Hohe Steinert 8 · 58509 Lüdenscheid · Germany · Phone +49 (0) 23 51/10 10 · Fax +49 (0) 23 51/10 12 17 · www.vossloh-schwabe.com
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Chemical Incompatibilities Affecting LEDs
 FURTHER POSSIBLY HARMFUL ENVIRONMENTS
In addition to chemical contamination due to process-related outgassing,
consideration must also be given to the environment in which the final
product will be installed and operated. All possible environmental conditions therefore have to be taken into account when designing an LED
luminaire (fixture). Aggressive environments of the kind detailed below can
negatively affect LED module behaviour and service life:
• Corrosive atmospheres (high sulphur dioxide content in the air: heavy
industrial environments)
• Coastal climates (salty air and high humidity)
• Chemical industry
• Environments with medium to high concentrations of hydrocarbons (e.g.
petrol stations)
• Acidic ambient air (e.g. medium to high concentrations of ammonia,
commonly found on livestock farms)
• Environments with medium to high concentrations of chlorine (e.g.
swimming pools)
• Radioactive and explosive environments
Chemical Name
Classification
Often found in ...
Siloxanes, fatty acids
Non-petroleum Oils
Silicone oil, lard, linseed
oil, castor oil
Sulphur compounds
Oxidizers/reducers
Gaskets, paints, sealants,
petroleum by-products
Cl, F,or Br containing
organic and inorganic
compounds
Halogenated hydrocarbons
Solder fluxes/pastes,
flame retardants
While chemicals in all three states (liquid, gaseous or solid) can be harmful to VS LED modules, gaseous chemicals (e.g. VOCs) can particularly
easily diffuse into silicone-based materials and cause LED arrays to
malfunction. However, the presence of VOCs is often not obvious. They
may form an undetected part of subcomponents used in a fixture or they
may have been used in early manufacturing steps, leaving traces of these
chemicals on different subcomponents of the finished products.
.
INFO
The list provided is not exhaustive and is intended only for
information. VS guarantees neither the completeness nor
validity of the list. Please treat the list only as a reference!
A much more extensive and detailed list of chemicals commonly used in
the production of lighting fixtures can be found in CREE’s application note
regarding the discolouration effect at www.cree.com.
Figure 16: Example of an LED module (WU-M-425) operated in an
environment with high concentrations of salt
Further information regarding the incompatibility of LED modules with
certain chemicals can also be found at the websites of Bridgelux, Philips,
Osram, Samsung, Xicato, Citizen or the Lighting Industry Federation (LIF).
 SUMMARY
 LIST OF INCOMPATIBLE CHEMICALS
Exposure of VS LED modules to the chemicals listed in the following table,
even in small quantities, should be avoided since they can have a harmful
effect on the light output and chromaticity of LED arrays.
Table 1: Chemicals and materials with known to be incompatible with
LEDs
Chemical Name
Classification
Often found in ...
Hydrochloric acid
Sulphuric acid
Nitric acid
Phosphoric acid
Acids
Cleaners, cutting fluids
Acetic acid
Organic acid
RTV silicones, cutting
fluids, degreasers,
adhesives
Sodium hydroxide
Potassium hydroxide
Amines
Alkali (Bases)
Detergents, cleaners
Ethers such as glycol ether
Ketones such as MEK, MIBK
Aldehydes such as formaldehyde
Organic Solvents
Cleaners, mineral
spirits, petroleum, paint,
gasoline
Xylene
Toluene
Benzene
Aromatic solvents
Cleaners, extracting or
pickling agents
Acetates
Acrylates
Aldehydes
Dienes
Low Molecular
weight Organics
(VOC’s)
Superglue, Loctite
adhesives, threadlockers
and activators, common
glues, conformal coatings, nail polish remover
Liquid hydrocarbons
Petroleum Oils
Machine oil, lubricants

IMPORTANT
Please note:
Irrespective of the specific LED manufacturer, the use
of incompatible materials can cause the sudden and
complete failure of LED modules.
• Careful consideration must be given to the materials and chemicals
used when processing VS LED modules and to such materials that
are embedded into the luminaire. Additional consideration must be
given to IP-rated (sealed) lighting fixtures with hermetic environments
enclosing the LED modules. Whereas environments with a good flow
of air let incompatible chemicals left from the manufacturing process
or gas diffusing out of materials found in the lighting fixture to escape,
a sealed system prevents the outgassing process and thus stops any
trapped incompatible chemicals from being vented out. This results in
LEDs being permanently exposed to contaminants, which in turn leads
to a pronounced and rapid discolouration of the module itself.
• Furthermore, consideration must also be given to the conditions in
which the final product will be installed and ultimately operated. Some
environments will have a highly harmful effect on LED modules and their
performance, resulting in a reduction of light output, shorter service life
and even, in some cases, complete module failure. In such cases, it
may be necessary to provide special protection to prevent LED modules coming into contact with any incompatible chemicals.
In general, VS advises against the use of any chemicals or materials
that have been found or are suspected to have an adverse effect on the
performance or reliability of LED devices.
Vossloh-Schwabe Deutschland GmbH · Hohe Steinert 8 · 58509 Lüdenscheid · Germany · Phone +49 (0) 23 51/10 10 · Fax +49 (0) 23 51/10 12 17 · www.vossloh-schwabe.com
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Chemical Incompatibilities Affecting LEDs
But ...
What can you do if using any of the listed chemicals in conjunction with
VS LED modules or if operating a product in a harmful environment proves
to be unavoidable?

To verify compatibility, VS recommends that all
chemicals and materials be tested on the entire fixture
under HTOL (High Temperature Operation Life)
conditions in the specific application and environment
for which they are intended.

End users of LED modules remain responsible for
ensuring sufficient air circulation inside the lighting
fixture and for preventing LED arrays from becoming
contaminated as a result of exposure to incompatible
substances or harmful environmental conditions that
may cause the LED module to malfunction or
damage the LED module.
IMPORTANT
IMPORTANT
Vossloh-Schwabe Deutschland GmbH · Hohe Steinert 8 · 58509 Lüdenscheid · Germany · Phone +49 (0) 23 51/10 10 · Fax +49 (0) 23 51/10 12 17 · www.vossloh-schwabe.com
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